The move_project script (T221112) does not support this: The selected child project is already a child of another project. This script can only move root-level projects beneath other projects, not move children within a hierarchy.
I guess I can
- create subproject Events Team 🡒 International-Developer-Events
- mass-move the currently 63 tasks under Community-Relations 🡒 International-Developer-Events (in theory via @Phabricator_maintenance but in practice T216867: @Phabricator_maintenance is sending email notifications might make you receive notifications)
- archive subproject Community-Relations 🡒 International-Developer-Events
Please confirm if that is wanted (merely asking because of notification spam).
- renamed old to-be-archived project, to allow creation of new subproject with the same name
- created subproject Events Team 🡒 International-Developer-Events at https://phabricator.wikimedia.org/project/view/4045/
- imported workboard columns for International-Developer-Events from the old to-be-archived project
- mass-moved the 63 tasks from old subproject to new one by using the @Phabricator_maintenance account: Remove old tag; Add new tag
- manually moved some tasks from Backlog column to workboard column they were in before - Phab obviously does not match columns with the same name :(
- updated project description of old subproject to point to new subproject at https://phabricator.wikimedia.org/project/profile/3455/
- archived old subproject
- updated project description of Community-Relations to not mention International-Developer-Events anymore
- spammed a lot of people with emails but you can filter and mass-delete by sender account Phabricator_maintenance
Closing as resolved.